Complex and multilevel left ventricular outflow tract obstruction: What can 3D echocardiography add?

Background Subaortic obstruction by a membrane or systolic anterior motion of the mitral valve leaflets is usually suspected in young patients, especially if the anatomy of the aortic valve is not clearly stenotic and unexplained left ventricular hypertrophy exists in the context of high transaortic gradients. Main body In certain circumstances, some patients show both aortic and subaortic stenotic lesions of variable severity. Doppler echocardiography can help in grading severity in the case of single-level obstruction but not in patients with multilevel obstruction where the continuity equation is of no value. Three-dimensional (3D) echocardiography allows "en-face" visualization of each level of the aortic valve and subaortic tract; in addition, direct planimetry of the areas can be done using multiplanar reformatting. Conclusions Accordingly, 3D echocardiography plays a crucial role in the assessment in patients with multilevel left ventricular outflow tract obstruction as it can accurately delineate the location and size, and severity of the stenosis. Supplementary Information The online version contains supplementary material available at 10.1186/s43044-021-00197-y.


Background
Obstruction of the left ventricular tract usually has variable etiologies and severity. It can be subaortic, aortic, or supra-aortic stenosis, single or multilevel stenosis [1][2][3]. The continuity equation is the most sensitive method in grading isolated aortic valve stenosis, provided that neither significant aortic valve regurgitation nor subaortic obstruction is present [4,5]. Subvalvular aortic stenosis may have a dynamic component and may be due to a fibrous membrane, muscular obstruction, or combined.  and causing nearly 300,000 deaths/year. Although it commonly affects the mitral valve, aortic valve can be affected in up to 30% of cases, resulting in serious effects on left ventricular function, quality of life, and overall prognosis. Nine percent of the patients have isolated aortic stenosis, 14% have isolated regurgitation, and 6% have mixed lesions. Rheumatic post-inflammatory lesions result in thickness, fibrosis of the aortic valve, and shrinkage of the cusps usually with fusion at the commissures and sometimes calcification [6][7][8].
Like the rheumatic mitral valve, the aortic valve shows systolic doming that makes proper planimetry of the narrowest orifice difficult to be done by the conventional two-dimensional (2D) transesophageal echocardiography (TOE). Moreover, coexisting mitral valve stenosis is commonly encountered. This can lead to paradoxically low gradients across the aortic valve [9][10][11][12].
3D echocardiography shows greater sensitivity in comparison with 2D TOE, especially with the use of multiplanar reformatting (MPR) of the 3D volumes and biplane mode. With the help of MPR, 3D-derived LVOT and aortic valve areas can be accurately traced.

Localization of the level of obstruction
Conventional 2D echocardiography allows the localization of the obstruction using Doppler echocardiography yet, it is not useful in patients with severe aortic valve regurgitation and in combined valvular and subvalvular stenosis where continuity equation will not be valid. Fig. 9 A case of rheumatic mitral and aortic valve diseases. In the upper panel, severe mitral valve stenosis was found; mean diastolic gradient = 15 mmHg .3D zoomed volume shows the characteristic "en-face" view of the thickened leaflets and commissural fusion. MVA was traced using MPR as 0.9 cm2. In the middle panel, a 2D parasternal long-axis view of the aortic valve shows systolic doming of the cusps. The apical 5-chamber view shows aortic valve regurgitation. In the lower panel, 3D-zoomed volume of the aortic valve shows thickened cusps and fused commissure between right and non-coronary cusps. X-plane mode and MPR-derived AVA are 0.9 cm2

Proposed protocol
With the advent of 3D echocardiography, planimetered areas of the aortic valve and LVOT can be easily traced.
It is recommended to start with 2D parasternal views with color Doppler to appreciate the aortic valve opening, presence of subaortic membrane and associated systolic anterior motion (SAM) of the mitral valve leaflets.
Apical 5-and 3-chamber views and right parasternal view if possible are of great value to measure the gradients across the LVOT (Fig. 1, Additional file 1: Video S1).
Using 2D TOE, aortic long-and short-axis mid-esophageal views and aortic transgastric views are more sensitive than transthoracic echocardiography (TTE) to clearly assess the above-mentioned data (Fig. 2).
3D-derived X-plane, zoomed and full-volume multibeat modes of acquisition are highly recommended to grade the severity of the stenosis. In the X-plane mode, TOE-derived aortic long-axis is obtained (nearly at ˜120 ˚), the line at the reference clip should be placed at the level of obstruction (Aortic cusps' tips or subaortic membrane) and then frozen the clip at the mid-systolic frame, and then, either the aortic valve area (AVA) or LVOT area can be easily traced in the corresponding perpendicular plane (Figs. 3, 4, Additional file 2 and 3: Video S2, S3).
Using zoomed and full-volume multi-beat modes, "enface" views of the aortic and subaortic planes can be easily obtained. The morphology and dynamic variations of each level are much easier and faster to be assessed in comparison with 2D TOE (Fig. 5, Additional file 4, 5 and 6: Video S4-S6).
Moreover, with routine exercising of MPR-derived measurements, areas of the valves and stenotic tracts can be easily traced as they are less susceptible to gain artifacts (Figs. 6,7,8,9,10,11,12,13, 14, Additional file 7 and 8: Video S7, S8). A stepwise approach for the assessment of aortic-mitral and aortic-subaortic stenoses is illustrated in Fig. 15. and subaortic stenosis is highly evolving with an accurate assessment of the size, location, and severity of each level of obstruction. Subsequently, 3D echocardiography is an excellent imaging tool to provide a roadmap before surgical interventions in such cases.  Stepwise approach for the assessment of aortic-mitral and aortic-subaortic stenoses